For controlling a combustion engine, it is known from DE 42 39 711 A1 (U.S. Pat. No. 5,558,178) to convert a desired value for a torque of the combustion engine into an actuating quantity for influencing the air supply to the combustion engine, for adjusting the ignition angle and/or for suppressing or switching in the fuel supply to individual cylinders of the combustion engine. Furthermore, it is additionally known from WO-A 95/24550 (U.S. Pat. No. 5,692,471) to influence the air/fuel ratio for realizing the pregiven torque value. Furthermore, in the known solutions, the actual torque of the internal combustion engine is computed while considering the instantaneous engine adjustment (charge, fuel metering and ignition angle). Here, the engine rpm, load (air mass, pressure, et cetera) and, if needed, the exhaust-gas composition are applied.
In the context of these computations, a torque model for the combustion engine is used which is used for determining the actuating quantities as well as for determining the actual quantities. The essence of this model is that an optimal torque of the combustion engine and an optimal ignition angle are determined in dependence upon an operating point. The optimal torque and optimal ignition angle are corrected by means of efficiency values in correspondence to the instantaneous adjustment of the combustion engine.
To optimize this model, it is provided in DE 195 45 221 A1 (U.S. Pat. No. 5,832,897) to correct the value for the optimal ignition angle in dependence upon quantities, which influence the degree of efficiency of the internal combustion engine. These quantities include the exhaust-gas recirculation rate, engine temperature, intake manifold air temperature, valve overlap angle, et cetera.
In practice, it has, however, been shown that this known solution can still be optimized, especially with respect to the simplicity of the application, the optimization of the computation time and/or the consideration of the operating-point dependency of the correction of the optimal ignition angle, especially, in dependence upon the inert gas rate. The known torque model shows unsatisfactory results in some operating states. Operating states of this kind are especially states having high inert gas rates in the combustion chamber, that is, states with a high component of inert gas (because of external or internal exhaust-gas recirculation), which are caused by overlapment of inlet and outlet valve opening times and which, above all, occur for low to medium fresh gas charges. Furthermore, these are operating states having a high charge movement. The computed base quantities lead to the situation that a precise torque computation is not achieved with the known procedure because these effects are not adequately considered.